The OptiX OSN 3500 forms three independent rings: north ring, southeast ring, and southwest ring. The fiber connection is unavailable between the three rings. Three sets of equipment on the three rings are respectively connected to the hub through a network cable. The hub is connected to the NM computer. Check the ECC routing. Log in to an NE on the north ring and query the ECC routing. It is found that the ECC can be connected to the equipment on the other two rings. The fiber connection, however, is unavailable between the three rings. Log in to an NE on the southeast and southwest rings respectively, and find the same fault exists. The extended ECC is disabled at the gateway NEs on the three rings. Normally, the NEs on the three rings cannot log in to each other in this case.